1. Field of the Invention
The present invention relates to an optical information recording medium in which information is recorded using laser light and the recorded information is reproduced by irradiating the recording medium with laser light and receiving the reflected light. More particularly, the present invention relates to a hybrid optical recording medium which has a ROM region in which data are preliminarily recorded by mastering and one of a recordable region in which additional information can be recorded and a recordable/erasable region in which information can be repeatedly recorded and erased. In addition, the present invention also relates to an information recording method using the information recording medium.
2. Discussion of the Related Art
In optical information recording media, information is recorded as follows:    (1) binarized digital data are recorded by forming concavo-convex pits on a spiral truck or concentric tracks (ROM discs) by embossing or the like;    (2) binarized digital data are recorded by forming pits in an inorganic or organic recording layer (recordable optical discs); and    (3) binarized digital data are recorded utilizing crystal changes of a recording layer (phase change optical discs).
When the recorded data are reproduced, a laser beam irradiates the track to receive the reflection light. The difference in strength of the reflection light is detected to reproduce the recorded data (i.e., to obtain the reproduced signals).
ROM discs, such as CDs, CD-ROMs and DVD-ROMs, in which information has been recorded, can be mass-produced at a low cost by transferring the concavo-convex pits formed on an original disc, to a plate. Such ROM discs are used for the distribution purpose. The concavo-convex pits can be formed on an original disc, for example, by the following method:    (1) a resist is formed on a glass plate or the like substrate;    (2) a laser light irradiates the resist to form a latent image; and    (3) the latent image is developed to form the concavo-convex pits on the glass plate.
In the above-mentioned recordable optical discs and phase change optical discs, such as CD-Rs, CD-RWs, DVD-Rs, DVD-RW, DVD+Rs and DVD+RWs, information recording is performed as follows:    (1) guide grooves for tracking are formed on a substrate by transferring the guide grooves formed on an original plate similarly to the method as mentioned above for use in the ROM discs;    (2) a recording material such as an inorganic or organic recording layer is formed on the substrate having guide grooves thereon; and    (3) information is recorded in the recording layer using a recording/reproducing apparatus such as CD-R drives.
It is required for the recordable discs such as recordable discs and phase change optical discs to reproduce the recorded information under the same reproducing conditions as those of ROM discs, i.e., information has to be reproduced similarly to the cases of CD-ROMs and CD-Rs (or CD-RWs), and DVD-ROMs and DVD-Rs (or DVD-RWs, DVD+Rs and DVD+RWs). This requirement can be fulfilled when the following is satisfied:    (1) information has to be recorded in recordable optical discs in the same format as that of ROM discs; and    (2) recorded signals to be reproduced have almost the same reproduction properties as those of ROM discs.
In the above-mentioned CD-Rs etc., address information is recorded in wobbled guide grooves, and recording pits, which are signals to be reproduced and which are similar to the pits of ROM discs, are formed by a recording drive.
ROM discs, which have both a recordable region and a ROM region therein, are called hybrid optical discs. The hybrid optical discs can be prepared by an injection method using a stamper. Therefore the hybrid optical discs can be easily mass-produced at a low cost. In addition, since the hybrid optical discs have a recordable region, mass-produced ROM data can be renewed and recordable media having software for controlling drives can be realized.
The present invention can be applied to hybrid optical discs (i.e., hybrid CD-R discs, hybrid CD-RW discs, hybrid DVD-R discs, hybrid DVD+R discs, hybrid DVD-RW discs and hybrid DVD+RW discs). In particular, the present invention is suitably applied to hybrid CD-R discs and hybrid CD-RW discs.
Japanese Patent Publication No. 7-114028 (i.e., Japanese Laid-Open Patent Publication No. 2-42652) discloses a hybrid CD-R medium having on the same plane both a ROM region including pits prepared by mastering and a recordable region formed on a groove portion. In the CD-R medium, the constitution of the ROM region is the same as that of normal CDs, and a reflection layer is directly formed on the pits of the substrate. However, it is difficult to selectively coat a dye on the groove portion without coating the dye on the ROM region. In addition, it is further difficult to coat a dye inside the ROM region. Therefore, a dye is coated on both the pit portion and groove portion while the pit form, groove form and dye coating conditions are optimized so that the signal characteristics of the pit portion and groove portion are optimized.
However, the push-pull signals from the ROM region are signals only output from the intermittent prepits, and the signals are much weaker than the push-pull signals (non-recorded signals), which are output from a continuous groove. Therefore, even when the modulation and push-pull amplitude (i.e., PPA), which are specified in the Orange Book, are fulfilled, the variation (hereinafter referred to as the in-plane variation) of the signals, push-pull/Ig (for a non-recorded portion) and push-pull/Iga (for the ROM region, i.e., recorded region) which are used for the tracking servo of recording apparatus, in each plane of the three zones often fall out of the range of ±3 dB (i.e., about ±40%). At this point, the three zones mean “the ROM region”, “a recorded portion of the RAM region” and “a non-recorded portion of the RAM region”. In particular, a problem which occurs is that tracking is not well performed by a drive in which the basic servo gain is set on the basis of the gain at the inner ROM region.
In addition, since the push-pull signals are weak in the ROM region, the wobble signals are also weak, resulting in occurrence of a problem in that ATIP (Absolute Time In Pre-groove) information cannot be read.
In order to solve this problem, it is known that an inter-pit groove is formed at a location between pits along the pit lines in the ROM region.
Japanese Laid-Open Patent Publication No. 5-36087 discloses an optical medium in which a wobbling groove (i.e., an inter-pit groove) which has the same form as that of a RAM groove is formed along the wobbling pits line in the ROM region. In this case, when the width of the RAM groove is set so as to be the same as the width of the inter-pit grooves, interference between the pits and the inter-pit grooves becomes large in an original plate preparation process, resulting in occurrence of sagging of the front and rear portions of the pits, and thereby a problem occurs in that the jitter at the ROM region becomes large. In addition, when the width of the inter-pit grooves is too narrow, a problem in that the wobble jitter becomes large occurs.
Japanese Laid-Open Patent Publication No. 9-138997 discloses an optical disc in which wobbling inter-pit grooves are formed to improve the characteristics of ATIP signals. However, the pits are not wobbled to lower the jitter at the ROM region. When an original plate of such an optical disc is prepared, two light beams are needed for forming the pits and inter-pit grooves. Therefore, the light irradiator used for forming the original plate has a complex optical device, and in addition, it is very hard to adjust the positions of the two light beams, i.e., it is hard to produce an original plate. Further, when the pits are not wobbled, a problem in that the wobble jitter becomes large occurs.